Asymmetrical digital subscriber line (ADSL) uses the twisted pair of the plain old telephone system (POTS) to transmit digital data. It uses the part of the line spectrum above the voice band and divides it into bins or channels as defined by the discrete multitione (DMT) standards of the International Telecommunication Union-Telecommunication (ITU-T).
DMT is just another name for orthogonal frequency-division multiplexing (OFDM). The modulation is quadrature amplitude modulation (QAM). Virtually all of the world's telephone companies implement ADSL for Internet access for phone customers, and they're now upgrading their systems to carry Voice over Internet Protocol as well as video.
ADSL describes a range of basic standards that support downstream data rates from 1.5 to 8 Mbits/s and upstream data rates up to 640 kbits/s. Its 18,000-ft maximum range is the most allowed in most POTS systems, but most local loops aren't that long. Of course, the length of the twisted pair definitely sets the upper data rate.
ADSL has been continuously developed and improved over the years with newer fast systems designated ADSL2, ADSL2+, and ADSL2++ (see the table). Other faster and symmetrical data-rate versions like high-speed symmetrical DSL (or SHDSL) also have been developed, though it isn't widely used. The newer, faster version of very high-data-rate DSL (VDSL) is being adopted in many areas.
Higher data rates are achieved by extending the upper bandwidth of the spectrum used on the cable and restricting the loop length. That plus improved filtering, pre-emphasis, and crosstalk mitigation techniques have further improved speed and range.